Spacecraft such as artificial satellites are intended to be stationed in orbit around a celestial body, in particular in Earth orbit, notably in geostationary orbit, in order to carry out their mission there, for example telecommunications, Earth observation, etc. Placing in station is usually carried out in two stages. The first stage consists in launching the satellite into space, notably from the surface of the Earth, by means of a specifically dedicated vehicle commonly called a launch vehicle, and injecting it into an initial orbit called the injection orbit. In the second stage, the satellite is transferred from this injection orbit to its mission orbit, also called its final orbit.
As is known in itself, in its mission orbit, notably in Earth orbit, a satellite is subjected to numerous disturbances. Those disturbances tend on the one hand to move the satellite relative to a set point position in its orbit and on the other hand to modify the attitude of said satellite relative to a set point attitude. In order to maintain the satellite substantially in the set point position and substantially in the set point attitude it is necessary to perform orbit control and attitude control of said satellite.
Orbit control consists in limiting variations of the orbital parameters. In the case of a GEO satellite, such as a telecommunication satellite, orbit control consists in controlling the position of the satellite relative to the Earth in terms of inclination, longitude and eccentricity, and is also known as station keeping (S/K).
Orbit control of a GEO satellite is generally effected by means of a plurality of orbit control maneuvers during which thrusters of the satellite are activated. Orbit control of the satellite is effected by adjusting the thrust forces produced by said thrusters during various orbit control maneuvers. A plurality of orbit control maneuvers are conventionally effected:                north/south (N/S) maneuvers enable control of the inclination of the orbit of the satellite,        east-west (E/W) maneuvers enable control of the longitude of the orbit of the satellite.        
For its part, eccentricity can be controlled during E/W maneuvers or N/S maneuvers.
It is possible to define a satellite frame of reference centered on a center of mass of this satellite and comprising three axes X, Y and Z: when the satellite is on station in its mission orbit, the axis X is parallel to a speed vector of the satellite, the axis Z is directed toward the Earth, and the axis Y is orthogonal to the axes X and Z. In the satellite frame of reference, N/S maneuvers necessitate thrust forces along the axis Y whereas E/W maneuvers necessitate thrust forces along the axis X. In the present description, the center of mass of the satellite means the theoretical center of mass of the satellite; its real center of mass can vary slightly over time as a function of the quantity of propellants in the tanks, the position/orientation of the equipments of the payload, etc.
Satellites are conventionally equipped with propulsion means able to perform on the one hand their transfer from the initial orbit to the mission orbit and on the other hand their station keeping in that mission orbit. These propulsion means can be of chemical type. Recently electrical propulsion has been used instead of chemical propulsion to carry out the transfer and station keeping of satellites because it enables better performance to be achieved compared to chemical propulsion.
Prior art satellites using electrical propulsion means employ electrical thrusters the thrust direction of which can be oriented by a mechanism. Orienting the thrusters notably makes it possible to control the position of the thrust direction relative to the center of mass of the satellite and to change from a configuration appropriate for the electrical transfer, in which all the thrusters are oriented along the same axis in the plane XZ (generally the axis Z of the satellite frame of reference) to a configuration appropriate for station keeping.
By way of example, the U.S. Pat. No. 5,443,231A describes a satellite comprising four electrical thrusters each mounted on a mechanism enabling orientation of its thrust direction. The orbital transfer and station keeping of this satellite are effected by simultaneous or sequential use of two thrusters disposed diagonally, both in nominal mode and in failure mode. However, a system of this kind is lacking in robustness, situations in which two thrusters disposed on the same side of the satellite become faulty being particularly penalizing for the mission of the satellite. Moreover, the maneuvers necessary for orbit and attitude control of such a satellite can prove complicated, notably where the control of eccentricity is concerned, and they are in particular liable to generate moments that can modify the attitude of the satellite, which it is necessary to remedy.